Unsaturated thermosetting polyester resins are widely used in industry today for the production of moldings, for example automobile parts molded from sheets, electrical breaker boxes, appliance housings, business machine housings, and a wide range of reinforced parts. These resins are polycondensation products which are based on the reaction of dibasic acids with dihydric alcohols. Ethylenic unsaturation is incorporated in the resin, generally by use of an unsaturated acid (e.g. maleic anhydride; but a saturated acid such as isophthalic may also be present). The alcohol is typically saturated, e.g. propylene glycol. The unsaturated polyesters are usually crosslinked through their double bonds with a compatible monomer (such as styrene) which also contains ethylenic unsaturation, and thereby become thermoset. The polyester itself is often blended with a thermoplastic resin for modified characteristics; for example, a polyvinyl acetate/styrene resin and/or an acrylic resin such as polymethacrylate/styrene resins are commonly included as "low profile additives" to improve surface smoothness. (As used herein, the term "polyester resin" is meant to include not only the polyester itself but also the crosslinking monomer which is conventionally added to the polyester and any other modifying resin that is blended with the polyester). The molding material may also include a conventional filler, a mold release agent and/or a thickener.
To effect crosslinking of the polyester, a catalyst of the peroxide type is used. The catalyst is generally premixed with the resin prior to molding and is thermally activated upon molding to initiate the crosslinking reaction. In such case curing does not begin until the mixture containing the catalyst is exposed to activating temperatures typically in the range of 300.degree. to 500.degree. F.; the optimum temperature to be used depends upon the specific nature of the catalyst, the duration of the molding cycle, and other factors. In general, higher temperatures above an optimum level effect more rapid crosslinking, but may impair product qualities.
In the molding of polyester resins, a certain minimum "cure time" is required to expose the molding to the curing conditions sufficiently to effect the cure. In high volume production, this cure time is, in effect, a "bottleneck," because it slows the rate of molding; other things being equal, the longer the cure time, the lower the production rate from a given mold. In order to increase output without using additional molding equipment, the reduction of cure time is a substantial economic advantage.